1. Field of the Invention
The invention relates in general to transport refrigeration systems, and more specifically to transport refrigeration systems in which the temperature of the air in each of two or more separate compartments of a trailer is to be controlled.
2. Description of the Prior Art
Transport refrigeration systems often utilize multi-temperature control for controlling the air temperature in two or more separate compartments of a multi-temperature trailer. For example, a trailer may carry a frozen load in the front compartment, which is controlled by the main refrigeration unit called the host unit, and fresh loads in the one or more remote compartments, which are controlled by remote evaporators. Some prior art systems utilize electric heaters supplied by an alternator to provide heat for a remote compartment. In order to provide the necessary alternator output, the dedicated prime mover, such as a Diesel engine, is operated at the higher of two normal operating speeds used at various times during the process of controlling the temperatures of the loads. For example, the higher of the two speeds may be 2200 RPM and the lower may be 1400 RPM. Operating the engine at high speed, however, when a rear compartment requires heat, often results in the front compartment getting extremely cold. While the compartments are separated and heat insulated from one another, the extreme cold in the forward compartment may leak to the adjacent compartment, which thus increases the need for heat in the adjacent compartment. The engine must thus run a longer time at the higher RPM to provide more heat for the rear compartment, while providing still more cold for the front compartment, and the system "fights" itself.
Hot gas from the host unit has been used to defrost the remote evaporator, but the use of hot gas to heat the remote evaporator has had disadvantages. For example, ambient temperatures below 40 degrees F. limit the heating capacity of the remote evaporator.
Remote evaporators have also suffered disadvantages not related to heating. For example, in systems which utilize a three-way valve to select host heating and cooling modes, the remote evaporator cannot go into a cooling mode when the host unit is in a heating mode, because the three-way valve is switched from a cooling position to a heating position by a pilot solenoid when the host unit requires heat.